1. Field of the Invention
The present invention relates to structures for natural draft cooling towers and, more particularly, to a lightweight supporting structure for a large natural draft cooling tower having a hyperboloid-shaped mantle.
2. Description of the Prior Art
The known cooling tower structures represent several different approaches to the problem of erecting a large vertical shell. One such approach features a steel structure consisting of straight structural steel profiles with flared portions which envelop the mantle area of the cooling tower, thereby creating a tower framework of hyperbolic outline. The mantle of the cooling tower consists of planks of wood or some other suitable material. This type of framework structure, using steel profiles as the only support, is comparatively costly as an investment and has the additional disadvantage that it requires considerable maintenance expense. Consequently, this approach has not prevailed, particularly in the case of larger cooling tower structures.
Other approaches to the construction of large cooling tower shells involve the use of reinforced concrete. One such approach provides for the cooling tower mantle to be cast with the aid of a mantle form which is being shifted in either a slip mode or a climbing mode or a rotational mode. Reinforcing irons are embedded in the mantle wall as the form is shifted from section to section.
Another known cooling tower structure (U.S. Pat. No. 3,304,351) requires the erection of a temporary scaffolding and the use of steel cables which are connected to the upper end of the scaffolding and tensioned downwardly against a ring beam which is to form the lower extremity of the cooling tower shell. The cables are oppositely diagonally inclined and tied together with circular tension rings, so as to form a hyperboloid-shaped cable network. The concrete mixture which is to form the cooling tower mantle is applied against this cable network in a spraying procedure. Once the concrete mantle is in place and hardened, the cables are released from the upper beams of the scaffolding and the latter is removed. The initial cable tension then becomes a compressive tension on the concrete shell, giving the cooling tower mantle its necessary stability and resistance.
The described concrete cooling tower structures are subject to high construction costs, due primarily to the complexity of the required scaffolding and concrete forms. An additional shortcoming relates to the fact that the available opening between the ground and the bottom edge of the cooling tower mantle, for reasons of static stability, cannot exceed a certain vertical distance. This is particularly undesirable in the case of very large cooling tower structures, the resultant limitation of the inlet cross section for the cooling air creating problems with respect to an even distribution of the cooling air over the cross-sectional area inside the cooling tower.
In the field of ventilator cooling towers, where tower dimensions are much smaller than the dimensions of natural draft cooling towers, there has been suggested a shell structure consisting of a cable grid to which the mantle is attached like a skin, using suitable lightweight mantle panels (U.S. Pat. No. 3,637,193). Circular spoked rings at the upper extremity and in the midportion of smallest cross section determine the shape of the cooling tower mantle, the spoked rings being fixedly connected to a central column which thereby carries the weight of the cooling tower mantle.
An important shortcoming of the described cablesupported structure is that its tensioning cables have to be angled off at the midportion of smallest diameter to such an extent that it becomes necessary to tension the cables separately in two length sections. In the conical upper section of the cooling tower, the cables need to be tensioned between the spoked ring on the exit extremity of the cooling tower and the spoked ring in the smallest-diameter midportion, thereby creating tensile forces in the cables which subject the spoked rings to vertical bending moments. In order to convert these bending moments into compressive forces which can be counteracted by the spokes of the two rings, it is necessary to arrange the spokes in an inclined radial orientation, the spokes coinciding with the surface of an imaginary cone. It has been found that these spokes need to be of very large cross section, especially in the case of large cooling towers, in order to sustain the combined stress which results from the cable forces and from the weight of the spoked ring and of the spokes themselves, given the considerable distance between the mantle and the central column. Numerous large spokes of this type, on the other hand, create a noticeable resistance to the flow of cooling air which, in turn, means a correspondingly higher power consumption of the ventilator or, in the case of a natural draft cooling tower, necessitates an increase in the height of the latter. Lastly, the spokes have a tendency to create a certain flow turbulence in the cooling air, resulting in an operating noise of considerable noise level.